Ozone in the troposphere is thought to determine the oxidizing power of an atmosphere and further to bring about the global warming and to exert adverse influences on living bodies. In recent years a rising tendency of the tropospheric ozone has been suggested, and it is of urgent necessity to make clear the mechanism of ozone formation and extinction on the global scale. In order to elucidate the mechanism of ozone formation and extinction, it is essential to verify the photochemical theory in a clean area which has not been affected by unnatural pollution.
Nitrogen oxides NOx (nitrogen monoxide NO and nitrogen dioxide NO2), which through photochemical reactions act as precursors of ozone and also are precursors of nitric acid, the substance forming acid rain, are thus extremely important species in the tropospheric photochemistry.
The concentration of nitrogen oxides in a clear atmosphere is thought to be several to several tens of pptv (the volume concentration unit in 10−12). Estimating the amount of formation of ozone in a clean area saved from air pollution requires a supersensitive nitrogen oxide concentration meter. Further, nitrogen dioxide and nitrogen monoxide are quickly transformed to each other by photochemical reactions in the daytime, and their time constants of transformation in the neighborhood of ground surface are generally around several minutes. It is thus necessary to hold down the measuring time to around 1 minute or less and it is required to complete the measurement in an extremely short period of time.
In the high-sensitivity measurement of the concentration of nitrogen oxides, use has so far been made of the chemiluminescence technique that detects a chmiluminescent light emitted when ozone reacts with nitrogen monoxide. If this technique is applied to measuring the concentration of nitrogen dioxide, nitrogen dioxide must for measurement of its concentration be first transformed to nitrogen monoxide through photodissociation or catalytic reaction. Further, this technique when used to complete the measurement in a measuring time of 1 minute has a limit of detection of 10 pptv, which is insufficient to measure the concentration of nitrogen dioxide in the clean atmosphere. Furthermore, the chemiluminescence technique, being an indirect method of measurement in which the concentration of nitrogen dioxide cannot be measured unless it is first transformed into nitrogen monoxide, has the shortcoming that it is affected largely by change and error in the conversion efficiency and also by alteration in the concentration of ozone monoxide in the atmospheric air.
There is also a technique in which molecules of nitrogen dioxide are excited by a laser light and the intensity of a fluorescent light emitted when the excited nitrogen dioxide molecules return to their ground state is measured to determine the concentration of nitrogen dioxide. Its principles are explained below.
A nitrogen dioxide molecule NO2 to transition from its ground state 2A1 to its excited state 2B2 absorbs a photon of frequency ν1 having the energy required for the transition to form a nitrogen dioxide molecule NO2* in the excited state, following the reaction:NO2*+hν1→NO2*   (1)
The nitrogen dioxide molecule NO2* in the excited state upon emitting a photon of frequency ν2 deviated towards red relative to frequency ν1 is returned to a nitrogen dioxide molecule NO2 in the ground state as follows:NO2*→NO2+hν2   (2)
Since the reaction (1) is proportional to the concentration of nitrogen dioxide molecules NO2, the number of photons of frequency ν2 is proportional to the concentration of nitrogen dioxide molecules NO2. It is thus possible to determine the concentration of nitrogen dioxide molecules NO2 by exciting the nitrogen dioxide molecules NO2 with a laser light and measuring the number of photons emitted by the nitrogen dioxide molecules NO2 in the excited state.
However, not only do those incident on a photon counting instrument include photons emitted in the form of a fluorescent light from nitrogen dioxide molecules, but also they include photons incident due to the scattering (Rayleigh scattering and Mie scattering) of the excited laser light by a particulate matter (aerosols) in the atmospheric air. Since the concentration to be measured here is in the pptv order of magnitude, an extremely feeble signal or a signal that is low in signal/noise ratio makes it impossible to accurately measure the concentration of nitrogen dioxide unless the scattered light intensity as a background noise is subtracted from the measured light intensity.
In addition to that due to the light scattering by the particulate matter, such background noises include those due to dark current from the photon detector and irregular reflections of light by instrument walls, which must also be subtracted in order to allow the concentration of nitrogen dioxide to be accurately determined.
A nitrogen dioxide concentration measurement apparatus designed to overcome these difficulties exists utilizing a dual wavelength laser induced fluorescence technique using a variable wavelength pulsed light laser. In this technique, two laser lights having their wavelengths corresponding to a peak and a valley of the absorption spectrum of nitrogen dioxide, respectively, are alternately applied for measurement. The first laser light of the wavelength corresponding to the peak of the absorption spectrum excites nitrogen dioxide molecules, and then there results a measure value that is the sum of the intensity of fluorescence emitted by nitrogen dioxide molecules, the intensity of scattered lights by the particulate matter and the other background noises. On the other hand, the second laser light of the wavelength corresponding to the valley of the absorption spectrum of nitrogen dioxide does not excite the nitrogen dioxide molecules, and then there results a measured value that is the sum of the scattered lights by the particulate matter and the other background noises.
It follows, therefore, that subtracting the measured value by the laser light of the wavelength corresponding to the valley from the measured value by the laser light of the wavelength corresponding the peak allows only the light intensity of the fluorescence from the nitrogen dioxide molecules to be known and thus the concentration of nitrogen dioxide to be determined.
According to this technique, however while it is made possible to measure a concentration of nitrogen dioxide at a level of several of pptv, such a variable wavelength laser becomes essential and indispensable that it can produce two laser lights having wavelengths adjacent to each other and corresponding to the peak and valley of the absorption spectrum of nitrogen dioxide, respectively, and further whose intensity ratio can be strictly controlled. Such a variable wavelength laser is not only expensive but also unstable in operation unavoidably by reason of its operating principles. In order to be made operable stably, it requires elaborate and large-scaled accessories, which are not suitable for an instrument that requires that measurements such as of global atmospheric environments be made simply and easily and regardless of places and seasons.
Under these circumstances, there has presently been no means or apparatus that allows the concentration of nitrogen dioxide as low as at a pptv level to be determined simply and easily and at the same time with high precision.